1. Field of the Invention.
The present invention relates to zinc-bromine batteries. More particularly, the present invention relates to a method of restoring activity to the electrodes within a zinc-bromine battery.
2. Description of the Prior Art.
A zinc-bromine battery is a type of bipolar, electrochemical flow battery. As with all batteries, a zinc-bromine battery is capable of collecting and discharging electric charge. A zinc-bromine battery typically includes a series or stack of voltaic cells, a pump for pumping electrolyte through the cells, terminal electrodes electrically coupled to the stack of cells, and stud terminals electrically coupled to the terminal electrodes and through which electric charge flows into and out of the battery.
The battery cells are made up of a series of alternating electrodes and separators. The electrodes are said to be "bipolar" because an anodic reaction takes place on one side of the electrode and a cathodic reaction takes place on its opposite side. Therefore, each cell can be considered as having an anodic half-cell and a cathodic half-cell. An ion permeable separator separates the anodic half-cell from the cathodic half-cell. Electrolyte is pumped through each half-cell; an anolyte through the anodic half-cell, and a catholyte through the anodic half-cell.
The electrolyte in zinc-bromine batteries is an aqueous solution of zinc-bromide and quaternary ammonium salts, for example, methylethylpyrrolidinium bromide, with optional supporting salts, such as NH.sub.4 Cl, and it is circulated through the individual cells from external reservoirs. It should be understood that the battery may be in several states including a discharged state and a charged state.
In the discharged state, the anolyte is substantially chemically identical to the catholyte. During the process of collecting a charge, the following chemical reaction takes place: EQU Zn++2 e.sup.- .fwdarw.Zn EQU 2 Br.sup.- .fwdarw.Br.sub.2 +2 e.sup.-
Zinc is plated on the anode, and bromine is produced at the cathode. The bromine is immediately complexed by the quaternary ammonium ions in the electrolyte to form a dense second phase which is subsequently removed from the battery stack with the flowing electrolyte. Further, and when the battery is charged, zinc in stored on one side of each electrode and the complex bromine is stored in the catholyte reservoir.
During the electrical discharge process, the following chemical reaction takes place. EQU Br.sub.2 +2 e.sup.- .fwdarw.2 Br.sup.-
Zn.fwdarw.Zn.sup.++ +2 e.sup.-
In this reaction, zinc is oxidized, and the released electrons pass through the electrode where they combine with molecular bromine to form bromide ions. Further, the positively charged zinc ions travel through the separator and remain in solution, and at the same time, bromide ions pass through the separator in the opposite direction and remain in solution.
Zinc-bromine batteries have several advantages over other types of batteries. In particular, one such advantage is the relatively high energy storage capacity of a zinc-bromine battery. Even though zinc-bromine batteries are in many ways superior to other types of batteries, they are not completely satisfactory. One problem in present zinc-bromine batteries is that the zinc-zinc ion and bromine-bromide reactions take place at different rates on the electrodes within the battery. The bromine-bromide reaction is relatively slow. The differential between the rates of reaction in the battery causes polarization which eventually causes battery failure. Polarization is a measure of internal voltage loss that occurs during discharge and charge of the battery.
One method of increasing the rate at which the bromine-bromide reaction takes place is to apply a high-surface-area carbon coating on the cathodic side of each electrode of a zinc-bromine battery. However, over time the rate at which the bromine-bromide reaction takes place on the carbon coated electrode decreases. In particular, over time the active surface area of the carbon coating decreases. In addition, oxidation of the carbon coating may occur. Therefore, present carbon coating techniques provide only a limited solution to the problems associated with the rate at which the bromine-bromide reaction occurs in a zinc-bromine battery.
What is needed, therefore, is a method to restore the activity to an electrode of a zinc-bromine battery. Further, what is needed is a method to restore the activity of an electrode of a zinc-bromine battery which will increase the performance and life expectancy of a zinc-bromine battery. More specifically, what is needed is a method to restore the activity of a bromine electrode, or the cathodic side of a bipolar electrode, of a zinc-bromine battery.